Failed to understand following angular code?

Question

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What is the difference between a normal function call and function returning promise and function returning observable and when and where we can use promise or observable??
difference between Http and httpclient??

JavaScript

getUserDataFiltered(searchText: string, pageNumber: number = 0, pageSize: number = 0, sortColumn: string = "", sortDirection: string = "", token: string) {
    return this.http.get(Constants.API_URL + 'admin/getUserData.php?searchText=' + searchText + '&pageNo=' + pageNumber + '&pageSize='
      + pageSize + "&sortColumn=" + sortColumn + "&sortDirection=" + sortDirection + '&token=' + token)
      .pipe(map(res => res.json()));
  } 

  deleteUser(adminLogin: string): Promise<string> {
    return this.http.post(Constants.API_URL + 'admin/deleteUser.php', adminLogin, this.options).toPromise()
      .then(this.extractData)
      .catch(this.handleErrorPromise);
  }

  readPolicies(): Observable<Policy[]>{
    return this.httpClient.get<Policy[]>('http://127.0.0.1/API/Policy/read.php');
  }

What I have tried:

I tried to google it and get the various answer on it but not statifed to any answer.

Posted 26-Sep-19 6:41am

Akshay malvankar

Updated 26-Sep-19 9:53am

Afzaal Ahmad Zeeshan

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Afzaal Ahmad Zeeshan · Answer 1 · 2019-09-26T09:53:00

They are both defined inside the Component that you are currently viewing, if you would just view their definition you will understand what they are. Also, Angular components have a great amount of offline documentation, so if you would just hover over the type (HttpClientModule, for instance) you will be provided with a complete documentation and use case of the Component... Which I recommend that you do for the next Component or Service that you want to learn more about. :-)

Read this[^], and you can understand what HttpClientModule was the latest update to Angular ecosystem, making HttpModule older.

See this too: Angular Http[^].
Angular - Http vs HttpClient[^]

One more thing, please use either one of them as it will cause version and API conflicts later on in the project. So, make a choice and remove the HttpModule (this.http) from the project and use the latest HttpClientModule.